The point people miss is that if you are taking £700 a week, then you can afford the fees. If your fees are £100 a month that sounds a lot, but to get to that level of fees, you're taking in the money.
You have to accept that because of the distance selling regulations that you do have to give refunds and suffer the postage cost loss. It's just a running cost - but compare that with a real shop rent/rates/insurance - despite the ebau and paypal fees, it's cheap!
I've never had a faulty product - I know my stuff is reliable and any problems are really down to the buyers other kit. This means they complain the goods are faulty, when they're not - despite putting warnings in the sales blurb like "Do not buy this if you do not have 48V phantom power available from your equipment" people still send messages saying it doesn't work. I ask what they plug it into, then look it up on Google and it's always the same answer. I tell them to send it back. One customer bought a very cheap product where the postage for the return would be more than the purchase price - I told him to keep it. No point worrying, just write off these sales and keep a 100% record. When I have to do a refund like this, instead of getting cross, I just wait for the next sale to roll in, and then say to myself - don't count this one, it wipes out yesterdays loss, and move on!
I cannot think of another outlet that can generate this business level.
On the under 18 front. Keep in mind that you've broken the law. They're granting you credit - hence the 18 requirement. If they find out, they have to cancel your account. They also pass your details to HMRC - all banks, building societies and finance houses (which covers Paypal) must provide the tax authority with details of any interest paid - so HMRC already know about your account.
Paypal will ignore your age as long as you don't raise your head above the parapet by being dodgy, but they're very good at spotting duplicate signups. I have never had a chargeback yet - through being as careful as I can be and as honest. They don't support you against a purchaser, they ALWAYS err on the side of the buyer, even if you provide cast iron proof. Live with it!